While working on MDL-40750, I noticed that using the keyboard to jump into courses inside a Combo box was not always working.

To replicate (on MacOS):

1. Have a category with at least a course inside
2. Add a combo box on the front page
3. Set maxcategorydepth to 1
4. Visit the front page and press "Tab"
5. Using the keyboard (or the mouse) expand one of the category
6. Try tabbing into the category content

Actual

• You cannot tab in the category content

Expected

• You can tab in the category and access the courses

This needs to be fixed for sub categories too

Checking at the code, it seems that it is on the first press of Tab that we add the tabindex, but if the categories are not yet expanded then tabindex will not be added. Why aren't we adding those tabindex in PHP if they are so important?

(This will affect 2.5 once MDL-40750 is integrated)